The present invention relates to high temperature transition joints and more particularly to such a joint adapted for interconnection between a first tubular part formed from a low alloy or carbon steel and a second tubular part formed from a high temperature alloy containing approximately 20% chromium.
In many high temperature applications, it is necessary to join together tubular parts of substantially different characteristics. For example, such applications arise in fossil-fired boiler construction and in nuclear power stations. In such power stations, high temperature joints are commonly required in various heat exchanger components such as boilers, steam generators, intermediate heat exchangers and recuperators, particularly in high temperature gas-cooled reactors, etc. Similar applications arise in other industries, such as petro-chemical equipment and chemical processing plants having substantial requirements for heat exchangers, steam lines and the like.
In all of these applications, it is usually necessary to form large numbers of tubular interconnections between materials of substantially different characteristics. Usually, one end of the tubular interconnection is formed from a high temperature alloy particularly suited for withstanding high temperature environments. At the same time, it is necessary for the interconnection or transition joint to withstand similar severe operating conditions of temperature, etc., over extended periods of time.
Under conditions of the type described above, the different types of materials to be interconnected by the transition joint exhibit substantially different physical characteristics making it difficult to maintain continuity throughout the transition joint. For example, the existence of very different thermal expansion rates on opposite sides of any given bond within such a high temperature joint tends to produce particularly severe stresses tending to cause cracking or total disruption of the bond. Other factors also exist within such transition joints which further interfere with the maintenance of an effective continuous transition joint or interconnection.
In many such high temperature applications, the different tubular materials to be interconnected consist of a low alloy steel or carbon steel on the one hand and a high temperature alloy composition on the other hand adapted to best withstand the severe high temperature conditions. Such compositions are particularly contemplated by the present invention and the high temperature alloy composition is further contemplated as containing approximately 16-20% chromium or more. Examples of such high temperature alloys include wrought or cast austenitic steels, such as ASTM or ASME Type 321H stainless or 304H stainless steel, and wrought or cast nickel base alloys. On the other hand, the low alloy, carbon containing steels may include ASTM or ASME SA213 Grade T22 steel containing for example 21/4% chromium, 1% molybdenum and 0.1% carbon, Grade T-11 steel containing approximately 11/4% chromium, 1/2% molybdenum and 0.1% carbon as well as other similar low alloy, carbon containing steels. The carbon steel may include ASTM or ASME SA-106, Grade A, B or C containing up to 0.35% carbon.
Substantial efforts have been expended in the past to develop effective transition joints for such applications. One such approach has been the formation of the joint with continuously changing chemical composition along the length of the joint, for example, by electroslag techniques, with one end of the joint being connected to one tubular piece and the other end of the joint joined to the other tubular piece of substantially different composition and characteristics. Many different types of material, such as powder metallurgy components and the like, have also been employed to form the transition joint. Heat treatment has also been employed both prior to and following formation of the transition joint in order to better condition the transition joint and interconnecting bonds to withstand severe operating conditions of the type referred to above. One particular problem encountered in such transition joints arises from the presence of low carbon or ferritic steel on one side of the joint and a high temperature alloy including a high chromium content on the other side of the joint. In such a situation, it has been found that the high chromium content exhibits a high affinity for carbon on the other side of the joint. Such a condition tends to cause migration of carbon across the bond from the low alloy steel, thereby producing a carbon-depleted zone immediately adjacent the bond which is particularly susceptible to disruption in the event of high thermal or mechanical stresses for example.
The prior art has been clearly aware of this particular problem, as noted for example by Zimmer U.S. Pat. No. 3,052,016. One prior art solution to this problem has been the use of a high nickel content in the part welded to the low alloy ferritic steel on the theory that the nickel would impede or limit carbon migration even in the presence of high chromium content. However, it has since been found that the use of high nickel does not prevent carbon migration over substantial periods of time. Thus, there remains a problem of providing an effective transition joint for such applications. Even with developments and improvements in the area of transition joints as discussed above, high failure rates have been and are being experienced with such transition joints exposed to severe operating conditions. Thus there has been found to remain a need for an improved transition joint to form an interconnection between low alloy or carbon steel tube or pipe and high temperature alloy tube or pipe including a high chromium percentage.